Method of fabricating an optoelectronic device having a bulk heterojunction

ABSTRACT

A method of fabricating an optoelectronic device comprises: depositing a first layer having protrusions over a first electrode, in which the first layer comprises a first organic small molecule material; depositing a second layer on the first layer such that the second layer is in physical contact with the first layer; in which the smallest lateral dimension of the protrusions are between 1 to 5 times the exciton diffusion length of the first organic small molecule material; and depositing a second electrode over the second layer to form the optoelectronic device. A method of fabricating an organic optoelectronic device having a bulk heterojunction is also provided and comprises: depositing a first layer with protrusions over an electrode by organic vapor phase deposition; depositing a second layer on the first layer where the interface of the first and second layers forms a bulk heterojunction; and depositing another electrode over the second layer.

This application is a continuation-in-part of U.S. application Ser. No.10/824,288, filed Apr. 13, 2004 entitled Method of Fabricating anOptoelectronic Device Having a Bulk Heterojunction, and which isincorporated by reference in its entirety.

UNITED STATES GOVERNMENT RIGHTS

This invention was made with U.S. Government support under Contract No.FA9550-04-1-0120 awarded by the U.S. Air Force Office of ScientificResearch and Contract No. ACQ-1-30619-05 (Prime DE-AC36-99G010337)awarded by the U.S. Department of Energy, National Renewable EnergyLaboratory. The government has certain rights in this invention.

RESEARCH AGREEMENTS

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a jointuniversity-corporation research agreement: Princeton University, TheUniversity of Southern California and Universal Display Corporation. Theagreement was in effect on and before the date the claimed invention wasmade, and the claimed invention was made as a result of activitiesundertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention generally relates to a method of fabricating anoptoelectronic device. More specifically, it is directed to a method offabricating an optoelectronic device including a bulk heterojunction.

BACKGROUND

Optoelectronic devices rely on the optical and electronic properties ofmaterials to either produce or detect electromagnetic radiationelectronically or to generate electricity from ambient electromagneticradiation. Photosensitive optoelectronic devices convert electromagneticradiation into electricity. Photovoltaic (PV) devices or solar cells,which are a type of photosensitive optoelectronic device, arespecifically used to generate electrical power. PV devices, which maygenerate electrical power from light sources other than sunlight, areused to drive power consuming loads to provide, for example, lighting,heating, or to operate electronic equipment such as computers or remotemonitoring or communications equipment. These power generationapplications also often involve the charging of batteries or otherenergy storage devices so that equipment operation may continue whendirect illumination from the sun or other ambient light sources is notavailable. As used herein the term “resistive load” refers to any powerconsuming or storing device, equipment or system. Another type ofphotosensitive optoelectronic device is a photoconductor cell. In thisfunction, signal detection circuitry monitors the resistance of thedevice to detect changes due to the absorption of light. Another type ofphotosensitive optoelectronic device is a photodetector. In operation aphotodetector has a voltage applied and a current detecting circuitmeasures the current generated when the photodetector is exposed toelectromagnetic radiation. A detecting circuit as described herein iscapable of providing a bias voltage to a photodetector and measuring theelectronic response of the photodetector to ambient electromagneticradiation. These three classes of photosensitive optoelectronic devicesmay be characterized according to whether a rectifying junction asdefined below is present and also according to whether the device isoperated with an external applied voltage, also known as a bias or biasvoltage. A photoconductor cell does not have a rectifying junction andis normally operated with a bias. A PV device has at least onerectifying junction and is operated with no bias. A photodetector has atleast one rectifying junction and is usually but not always operatedwith a bias.

Traditionally, photosensitive optoelectronic devices have beenconstructed of a number of inorganic semiconductors, e.g., crystalline,polycrystalline and amorphous silicon, gallium arsenide, cadmiumtelluride and others. Herein the term “semiconductor” denotes materialswhich can conduct electricity when charge carriers are induced bythermal or electromagnetic excitation. The term “photoconductive”generally relates to the process in which electromagnetic radiant energyis absorbed and thereby converted to excitation energy of electriccharge carriers so that the carriers can conduct, i.e., transport,electric charge in a material. The terms “photoconductor” and“photoconductive material” are used herein to refer to semiconductormaterials which are chosen for their property of absorbingelectromagnetic radiation to generate electric charge carriers.

More recently, optoelectronic devices that make use of organic materialshave become increasingly desirable for a number of reasons. Many of thematerials used to make such devices are relatively inexpensive, soorganic optoelectronic devices have the potential for cost advantagesover inorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Organic materials have manufacturing process advantages relative toinorganic thin-film technologies enabling the ability to apply organicmaterial layers to plastic substrates. Examples of organicoptoelectronic devices include organic light emitting devices (OLEDs),organic phototransistors, organic photovoltaic cells (OPVs), and organicphotodetectors. As used herein, the term “organic” includes polymericmaterials as well as small molecule organic materials that may be usedto fabricate organic optoelectronic devices. “Small molecule” refers toany organic material that is not a polymer, and “small molecules” mayactually be quite large. Small molecules may include repeat units insome circumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. For OLEDs, the organicmaterials therein may have performance advantages over conventionalmaterials. For example, the wavelength at which an organic emissivelayer emits light may generally be readily tuned with appropriatedopants. Several OLED materials and configurations are described in U.S.Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporatedherein by reference in their entireties.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in an organic optoelectronic device. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated herein by reference in their entireties, may also be used.For a device intended to emit light only through the bottom electrode,the top electrode does not need to be transparent, and may be comprisedof a thick and reflective metal layer having a high electricalconductivity. Similarly, for a device intended to emit light onlythrough the top electrode, the bottom electrode may be opaque and/orreflective. Where an electrode does not need to be transparent, using athicker layer may provide better conductivity, and using a reflectiveelectrode may increase the amount of light emitted through the otherelectrode, by reflecting light back towards the transparent electrode.Fully transparent devices may also be fabricated, where both electrodesare transparent. Side emitting OLEDs may also be fabricated, and one orboth electrodes may be opaque or reflective in such devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” or “deposited over” a secondlayer, the first layer is disposed or deposited further away from thesubstrate. There may be other layers between the first and second layer,unless it is specified that the first layer is “in physical contactwith” the second layer. For example, a cathode may be described as“disposed over” an anode, even though there are various organic layersin between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

Solar cells can be characterized by the efficiency with which they canconvert incident solar power to useful electric power. Devices utilizingcrystalline or amorphous silicon dominate commercial applications, andsome have achieved efficiencies of 23% or greater under solarillumination. However, efficient crystalline-based devices, especiallyof large surface area, are difficult and expensive to produce due to theproblems inherent in producing large crystals without significantefficiency-degrading defects. On the other hand, high efficiencyamorphous silicon devices still suffer from problems with stability.Present commercially available amorphous silicon cells have stabilizedefficiencies between 4 and 8%. Due to the manufacturing and stabilityissues associated with crystalline or amorphous silicon devices, thecost of the electrical power produced per unit area has been high. Theuse of current inorganic solar cells is currently limited by their highmanufacturing costs relative to the electrical power produced per unitarea, as well as their high installation costs resulting from thecumbersome nature of the cell frames, the remainder of system equipment,and the panels themselves. Thus, a need exists for a more economicalmethod of manufacturing PV cells having an acceptable quantum yield andtherefore, power conversion efficiency.

More recent efforts have focused on the use of organic photovoltaiccells to achieve acceptable photovoltaic conversion efficiencies witheconomical production costs. In organic materials, light absorptionleads to the formation of excitons, or bound electron-hole pairs, ratherthan the free electron-hole pairs produced in inorganic semiconductors.Efficient dissociation of excitons in organic materials occurs understrong electric fields, or at a donor-acceptor (DA) heterointerfacewhere the differences in the electron affinities and the ionizationpotentials between the contacting organic materials are sufficientlylarge to overcome the exciton binding energy. The later mechanism hasbeen employed to form an organic DA planar heterojunction (HJ)photovoltaic cell with a power conversion efficiency 0_(P)˜1%, which waslimited by exciton diffusion lengths (L_(D) # 100 Angstroms) being muchshorter than the optical absorption length (L_(A)˜1000 Angstroms).Employing C₆₀ (see U.S. Pat. No. 6,580,027) as the acceptor material,which has L_(D)˜400 Angstroms, a power conversion efficiency 0_(P)˜3.6%has been demonstrated. Because excitons generated within an excitondiffusion length of the DA heterojunction are subject to the electricfield of the junction, they generally have a higher probability ofdissociating efficiently. Conversely, excitons generated more than anexciton diffusion length from the DA heterojunction generally have asmaller probability of dissociating efficiently and contributing todevice current. Bulk heterojunctions seek to create a highly interfoldedor interpercolating network of the donor and acceptor materials suchthat an exciton generated by the absorption of incident radiation islikely close to a heterojunction and is likely to disassociateefficiently contributing to device current.

SUMMARY OF THE INVENTION

The present invention provides a method of fabricating an organicoptoelectronic device. Examples of organic optoelectronic devices whichmay be fabricated by the method of the invention include, but are notlimited to, organic light emitting devices (OLEDs), organicphototransistors, organic photovoltaic cells (OPVs) (or solar cells),and organic photodetectors.

According to an embodiment of the present invention, a method offabricating an optoelectronic device comprises the steps of: depositinga first layer having protrusions over a first electrode, in which thefirst layer comprises a first organic small molecule material;depositing a second layer on the first layer such that the second layeris in physical contact with the first layer; in which the smallestlateral dimension of the protrusions is between 1 to 5 times the excitondiffusion length of the first organic small molecule material; anddepositing a second electrode over the second layer to form theoptoelectronic device. In another embodiment, the spacing between theprotrusions is between 1 to 5 times the exciton diffusion length of thesecond layer.

The present invention also provides a method of fabricating an organicoptoelectronic device having a bulk heterojunction.

According to an embodiment of the present invention, a method offabricating an optoelectronic device comprises the steps of: depositinga first layer with protrusions over a first electrode by organic vaporphase deposition, in which the first layer comprises an organic smallmolecule material and the smallest lateral dimension of the protrusionsis at least 5% less than the thickness of the device; depositing asecond layer on the first layer such that the second layer is inphysical contact with the first layer, and in which the interface of thefirst and second layers forms a bulk heterojunction; and depositing asecond electrode over the second layer to form the optoelectronicdevice. For example, when the first layer is an electron donor layer,the first electrode is an anode, the second layer is an electronacceptor layer, and the second electrode is a cathode. As a furtherexample, when the first layer is an electron acceptor layer, the firstelectrode is a cathode, the second layer is an electron donor layer, andthe second electrode is an anode.

According to another embodiment of the present invention, a method offabricating an optoelectronic device comprises the steps of: depositinga first layer having protrusions over a first electrode, wherein thefirst layer comprises a first organic small molecule material;depositing a second layer on the first layer such that the second layeris in physical contact with the first layer, wherein the interface ofthe second layer on the first layer forms a bulk heterojunction; anddepositing a second electrode over the second layer to form theoptoelectronic device. For example, when the first layer is an electrondonor layer, the first electrode is an anode, the second layer is anelectron acceptor layer, and the second electrode is a cathode. As afurther example, when the first layer is an electron acceptor layer, thefirst electrode is a cathode, the second layer is an electron donorlayer, and the second electrode is an anode.

It is an object of the present invention to provide a method for thefabrication of an optoelectronic device including a bulk heterojunction,wherein the method is conducive to continuous and so-called“roll-to-roll” manufacturing techniques, and results in optoelectronicdevices having acceptable quantum yields and power conversionefficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematic diagrams of the following types of organicdonor-acceptor photovoltaic cells: (a) a bilayer cell; (b) a bulkheterojunction cell; and (c) a mixed-layer cell.

FIG. 2 shows a schematic diagram of organic vapor phase deposition(OVPD). OVPD takes place inside of a heated chamber and proceeds inthree steps: evaporation from the source cell, entrainment of organicvapor by the carrier gas toward the substrate, and diffusion across theboundary layer to adsorb onto the substrate. The hot chamber walls anddirected gas flow increase the deposition efficiency, prevent long-termcontamination, and improve doping control.

FIG. 3 shows scanning electron micrographs of the following layersdeposited by organic vapor phase deposition: (a) CuPc deposited onsilicon at an underlying substrate temperature of about 60° C.; (b) CuPcdeposited on silicon at an underlying substrate temperature of about100° C.; and (c) CuPc deposited on ITO at an underlying substratetemperature of about 100° C.

FIG. 4 shows a schematic diagram of an optoelectronic device produced byan embodiment of the method of the present invention.

FIG. 5 shows a scanning electron microscope image of the surface of CuPcfilm with a continuous wetting layer and needle-like crystals grown on asilicon wafer by OVPD.

FIG. 6 shows a scanning electron microscope image of the surface of aCuPc film grown on iridium tin oxide coated glass by OVPD.

FIG. 7 shows a scanning electron microscope image of the surface of aCuPc film grown on iridium tin oxide coated glass by OVPD. The inset isan atomic microscope image of the same sample.

FIG. 8 shows a scanning electron microscope image of the surface of a500 thick CuPc film grown on iridium tin oxide coated glass by vacuumthermal evaporation. The inset is an atomic microscope image of the samesample.

FIG. 9( a) shows a Bragg-Brentano X-ray diffractrogram of CuPc filmgrown by OVPD on iridium tin oxide coated substrate. FIG. 9( b) shows aBragg-Brentano X-ray diffractrogram of CuPc film grown by VTE on iridiumtin oxide coated substrate

FIG. 10 shows a schematic diagram of the conformal surface and shadowingeffects that result from ballistic trajectories followed by the incidentmolecules in vacuum thermal evaporation.

FIG. 11 shows a scanning electron microscope image of the surface of aPTCBI/BCP/Ag film grown by vacuum thermal evaporation over CuPc needletrays.

FIG. 12 shows a schematic diagram of the planar surface and gap-fillingcharacteristic of the top film grown in OVPD that results from increasedsurface diffusion and random arrival directions of molecules.

FIG. 13 shows a scanning electron microscope image of the surface of aPTCBI layer grown over CuPc needles.

FIG. 14 shows the current-voltage characteristics of an OVPD growncontrolled CuPc/PTCBI bulk heterojunction photovoltaic cell at variousincident power levels.

FIG. 15 shows the power conversion efficiency, η_(p), open-circuitvoltage, V_(oc), and fill factor, FF, as functions of an incident powerintensity P_(o) for an OVPD grown controlled CuPc/PTCBI bulkheterojunction device.

DETAILED DESCRIPTION

When electromagnetic radiation of an appropriate energy is incident upona semiconductive organic material, for example, an organic molecularcrystal (OMC) material, or a polymer, a photon can be absorbed toproduce an excited molecular state. This is represented symbolically asS₀+hνΨS₀*. Here S₀ and S₀* denote ground and excited molecular states,respectively. This energy absorption is associated with the promotion ofan electron from a bound state in the HOMO, which may be a B-bond, tothe LUMO, which may be a B*-bond, or equivalently, the promotion of ahole from the LUMO to the HOMO. In organic thin-film photosensitiveoptoelectronic material, the generated molecular state is generallybelieved to be an exciton, i.e., an electron-hole pair in a bound statewhich is transported as a quasi-particle. The excitons can have anappreciable life-time before geminate recombination, which refers to theprocess of the original electron and hole recombining with each other,as opposed to recombination with holes or electrons from other pairs. Toproduce a photocurrent the electron-hole pair must become separated,typically at a donor-acceptor interface between two dissimilarcontacting organic thin films. If the charges do not separate, they canrecombine in a geminant recombination process, also known as quenching,either radiatively, by the emission of light of a lower energy than theincident light, or non-radiatively, by the production of heat. Either ofthese outcomes is undesirable in a photosensitive optoelectronic device.

Electric fields or inhomogeneities at a contact may cause an exciton toquench rather than dissociate at the donor-acceptor interface, resultingin no net contribution to the current. Therefore, it is desirable tokeep photogenerated excitons away from the contacts. This has the effectof limiting the diffusion of excitons to the region near the junction sothat the associated electric field has an increased opportunity toseparate charge carriers liberated by the dissociation of the excitonsnear the junction.

To produce internally generated electric fields which occupy asubstantial volume, the usual method is to juxtapose two layers ofmaterial with appropriately selected conductive properties, especiallywith respect to their distribution of molecular quantum energy states.The interface of these two materials is called a photovoltaicheterojunction. In traditional semiconductor theory, materials forforming PV heterojunctions have been denoted as generally being ofeither n-type or p-type. Here n-type denotes that the majority carriertype is the electron. This could be viewed as the material having manyelectrons in relatively free energy states. The p-type denotes that themajority carrier type is the hole. Such material has many holes inrelatively free energy states. The type of the background, i.e., notphoto-generated, majority carrier concentration depends primarily onunintentional doping by defects or impurities. The type andconcentration of impurities determine the value of the Fermi energy, orlevel, within the gap between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO), called theHOMO-LUMO gap. The Fermi energy characterizes the statistical occupationof molecular quantum energy states denoted by the value of energy forwhich the probability of occupation is equal to ½. A Fermi energy nearthe LUMO energy indicates that electrons are the predominant carrier. AFermi energy near the HOMO energy indicates that holes are thepredominant carrier. Accordingly, the Fermi energy is a primarycharacterizing property of traditional semiconductors and theprototypical PV heterojunction has traditionally been the p-n interface.

The term “rectifying” denotes, inter alia, that an interface has anasymmetric conduction characteristic, i.e., the interface supportselectronic charge transport preferably in one direction. Rectificationis associated normally with a built-in electric field which occurs atthe heterojunction between appropriately selected materials.

In the context of organic materials, the terms “donor” and “acceptor”refer only to the relative positions of the HOMO and LUMO energies oftwo contacting but different organic materials. If the LUMO energy ofone material in contact with another is higher (i.e., its distance tothe vacuum level is smaller), then that material is a donor. If the HOMOof one material relative to another is lower (same definition, but theHOMO is directly measured by the ionization potential, which is smallerfor the lower HOMO), then that material is an acceptor.

A significant property in organic semiconductors is carrier mobility.Mobility measures the ease with which a charge carrier can move througha conducting material in response to an electric field. As opposed tofree carrier concentrations, carrier mobility is determined in largepart by intrinsic properties of the organic material such as crystalsymmetry and periodicity. Appropriate symmetry and periodicity canproduce higher quantum wavefunction overlap of HOMO levels producinghigher hole mobility, or similarly, higher overlap of LUMO levels toproduce higher electron mobility. Moreover, the donor or acceptor natureof an organic semiconductor, e.g., 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA), may be at odds with the higher carrier mobility.For example, while chemistry arguments suggest a donor-type characterfor PTCDA, experiments indicate that hole mobilities exceed electronmobilities by several orders of magnitude so that the hole mobility is acritical factor. The result is that device configuration predictionsfrom donor/acceptor criteria may not be borne out by actual deviceperformance. Due to these unique electronic properties of organicmaterials, rather than designating them as “n-type” or “p-type,” thenomenclature of “hole-transporting-layer” (HTL) or “donor,” or“electron-transporting-layer” (ETL) or “acceptor” is frequently used. Inthis designation scheme, an ETL will be preferentially electronconducting and an HTL will be preferentially hole transporting.

A typical prior art photovoltaic device configuration is the organicbilayer cell, a schematic diagram of which is shown in FIG. 1( a). Inthe bilayer cell, charge separation predominantly occurs at the organicheterojunction. The built-in potential at the heterojunction isdetermined by the HOMO-LUMO energy difference between the two materialscontacting to form the heterojunction. The HOMO-LUMO gap offset betweenthe donor and acceptor materials produce an electric field at thedonor/acceptor interface that facilitates charge separation for excitonscreated within an exciton diffusion length of the interface.

The power conversion efficiency, 0_(P), of both small molecular weightand polymer organic photovoltaic cells has increased steadily in thelast decade. This progress may be, to a great extent, attributed to theintroduction of the donor-acceptor (DA) heterojunction which functionsas a dissociation site for the strongly bound photogenerated excitons.Further progress was realized in polymer devices through use of blendsof the donor and acceptor materials. Phase separation duringspin-coating leads to a bulk heterojunction which removes the excitondiffusion bottleneck by creating an interpenetrating network of thedonor and acceptor materials. However, the realization of bulkheterojunctions using mixtures of vacuum-deposited small molecularweight materials has been elusive since phase separation, induced byelevating the substrate temperature, leads to a significant rougheningof the film surface and short-circuited devices.

The external quantum efficiency of an organic photosensitiveoptoelectronic device is based on exciton dissociation at a DAinterface, i.e., 0_(EQE)=0_(A). 0_(ED). 0_(CC). Here, 0_(A) is theabsorption efficiency. The diffusion efficiency, 0_(ED), is the fractionof photogenerated excitons that reaches a DA interface beforerecombining. The carrier collection efficiency, 0_(CC), is theprobability that a free carrier, generated at a DA interface bydissociation of an exciton, reaches its corresponding electrode.Typically, in bilayer DA photovoltaic cells with a total thickness, L,on the order of the optical absorption length, L_(A), we have0_(A)=1−exp(−L=L_(A))>50% if optical interference effects are ignored,and 0_(CC). 100%. However, since the exciton diffusion length (L_(D)) inorganic materials is typically an order of magnitude smaller than L_(A),a large fraction of the photogenerated excitons remains unused forphotocurrent generation (see FIG. 1( a)). This provides a significantlimit to 0_(EQE) and hence 0_(P) of this type of planar junction cell.Conversely, when L is less than or equal to L_(D), 0_(ED)=100% and thus0_(EQE) is then limited by 0_(A).

In polymer photovoltaic cells, the exciton diffusion bottleneck has beenremoved through the introduction of a bulk heterojunction. A schematicdiagram of a bulk heterojunction is shown in FIG. 1( b). In a bulkheterojunction, the DA interface is highly folded such thatphotogenerated excitons are likely to find a DA interface within adistance L_(D) of their generation site. Currently, state-of-the-artbulk heterojunction polymer photovoltaic cells have power conversionefficiencies of up to 3.5%.

The bulk heterojunction has been typically fabricated by (1)spin-coating a mixture of soluble versions of the donor and acceptormaterials and (2) phase segregation of a mixture of a donor/acceptormaterial by high temperature annealing of small molecular-weight organiclayers. During spin coating and solvent evaporation, the donor andacceptor materials phase separate, creating an intricateinterpenetrating network with a large interfacial area between the twophases. The morphology of the resulting structure is controlled bychanging the spin conditions, solvents and relative materialconcentrations. The challenge of such systems is to balance a high0_(ED), favoring finely grained morphologies, and a high 0_(CC) favoringcoarse granularity, such that the product 0_(ED). 0_(CC) is maximized.The method of annealing-induced phase segregation is described inPeumans, Uchida, & Forrest, “Efficient bulk heterojunction photovoltaiccells using small-molecular-weight organic thin films,” Nature 425(2003) 158-162.

Currently, realizations of bulk heterojunctions in small molecularsystems have been largely unsuccessful. Attempts to achieve a bulkheterojunction through co-deposition of the donor and acceptor materialshave yielded devices with power conversion efficiencies falling short ofthose achievable in optimized bilayer devices using the same materials.Strong quenching of the photoluminescence in mixed materials indicatesthat 0_(ED)˜100%. Therefore, the low efficiencies are attributed to poorcharge transport, resulting in low carrier collection efficiencies,0_(CC) (see FIG. 1( c)). If charge collection is assisted by theapplication of an external voltage, high external quantum efficienciescan be obtained.

Growth of mixed layers at elevated substrate temperatures leads to phaseseparation and the appearance of crystalline domains. However, thisincrease in crystallinity and possibly larger L_(D) comes at the cost ofan increased film roughness. The high density of pinholes leading toshort circuits between cathode and anode contacts in such structuresmakes device fabrication impractical. The same problem occurs whenmixed-layer films are annealed post-deposition to induce phaseseparation.

Moreover, although bulk heterojunctions produced by the above methodshave resulted in increased power conversion efficiency, these bulkheterojunctions also exhibit high series resistance due to thedisordered structure of the interface layer. Phase separation duringspin-coating of polymers and annealing-induced phase segregation areboth “thermodynamically driven” methods characterized by randomlystructured interdigitation between the donor and acceptor layers due tothe entropy of the interface formation process.

In accordance with the present invention, a method is provided for thefabrication of an organic optoelectronic device having a bulkheterojunction in a small molecule system. More specifically, the methodalso provides for controlled grown of an ordered bulk heterojunction.According to an embodiment of the invention, a first layer comprising anorganic small molecule material is deposited over a first electrode byorganic vapor phase deposition (OVPD). It is believed that this methodproduces bulk heterojunctions with high surface area-to-volume ratio andreduced series resistance as compared with other known methods. Asillustrated in Table I, and further discussed below, it has been shownthat η_(P) for devices with bulk heterojunctions fabricated by OVPDaccording to embodiments of the invention is approximately 2.5 timeshigher than achieved using a comparable planar heterojunction PV cell,and 1.9 times higher than an annealed, thermodynamically driven bulkheterojunction. Thus, it is believed that the controlled growth of thepreferred embodiment of CuPc/PTCBI bulk heterojunction results in thedesired increase in junction surface area without introducing anincreased cell series resistance.

Typically, the thin films of organic optoelectronic devices, such asOLEDs and OPVs, are grown by thermal evaporation in high vacuum,permitting the high degree of purity and structural control needed forreliable and efficient operation (see S. R. Forrest, Chem. Rev. 97, 1793(1997)). However, control of film thickness uniformity and dopantconcentrations over large areas needed for manufactured products can bedifficult when using vacuum evaporation (see S. Wolf and R. N. Tauber,Silicon Processing for the VLSI Era (Lattice, 1986)). In addition, aconsiderable fraction of the evaporant coats the cold walls of thedeposition chamber. Over time, inefficient use of materials results in athick coating which can flake off, leading to particulate contaminationof the system and substrate. The potential throughput for vacuumevaporated organic thin film devices is low, resulting in highproduction costs. Low-pressure organic vapor phase deposition (LP-OVPD)has been demonstrated recently as a superior alternative technique tovacuum thermal evaporation (VTE), in that OVPD improves control overdopant concentration of the deposited film, and is adaptable to rapid,particle-free, uniform deposition of organics on large-area substrates(see M. A. Baldo, M. Deutsch, P. E. Burrows, H. Gossenberger, M.Gerstenberg, V. S. Ban, and S. R. Forrest, Adv. Mater. 10, 1505 (1998)).

OVPD is inherently different from the widely used vacuum thermalevaporation (VTE), in that it uses a carrier gas to transport organicvapors into a deposition chamber, where the molecules diffuse across aboundary layer and physisorb on the substrate. This method of filmdeposition is most similar to hydride vapor phase epitaxy used in thegrowth of III-V semiconductors (see G. B. Stringfellow, OrganometallicVapor-Phase Epitaxy (Academic, London, 1989); G. H. Olsen, in GaInAsP,edited by T. P. Pearsall (Wiley, New York, 1982)). In LP-OVPD, theorganic compound is thermally evaporated and then transported through ahot-walled gas carrier tube into a deposition chamber by an inertcarrier gas toward a cooled substrate where condensation occurs. Flowpatterns may be engineered to achieve a substrate-selective, uniformdistribution of organic vapors, resulting in a very uniform coatingthickness and minimized materials waste.

Virtually all of the organic materials used in thin film optoelectronicdevices have sufficiently high vapor pressures to be evaporated attemperatures below 400° C. and then to be transported in the vapor phaseby a carrier gas such as argon or nitrogen. This allows for positioningof evaporation sources outside of the reactor tube (as in the case ofmetalorganic chemical vapor deposition (see S. Wolf and R. N. Tauber,Silicon Processing for the VLSI Era (Lattice, 1986); G. B. Stringfellow,Organometallic Vapor-Phase Epitaxy (Academic, London, 1989))), spatiallyseparating the functions of evaporation and transport, thus leading toprecise control over the deposition process.

The concept of OVPD is illustrated in FIG. 2. The vapors of species Aare generated by heating the source material in a stream of an inertcarrier gas, which transports the vapors into the deposition chamber.The gas flow around the substrate forms a hydrodynamic boundary layer,across which molecules (in typical concentrations of <0.01%) diffuse andadsorb onto the substrate. This process can be analyzed as a sequence ofdistinct transport steps. In conjunction with the dynamics of adsorptionof organic species A on the substrate, these steps may be summarized as:

$\begin{matrix}{{Evaporation}\text{:}\mspace{14mu} A_{s}{\prod\limits^{k_{evap}}\; A_{g}}} & \left( {1a} \right) \\{{Re}\text{-}{condensation}\text{:}\mspace{14mu} A_{g}{\prod\limits^{k_{cond}}\; A_{s}}} & \left( {1b} \right) \\{{Entrainment}\mspace{14mu}{by}\mspace{14mu}{carrier}\mspace{14mu}{gas}\text{:}\mspace{14mu} A_{g}{\prod\limits^{k_{pickup}}\; A_{cg}}} & \left( {1c} \right) \\{{Transport}\mspace{14mu}{to}\mspace{14mu}{the}\mspace{14mu}{substrate}\text{:}\mspace{14mu} A_{cg}{\prod\limits^{k_{t}}\; A_{{cg},{subs}}}} & (2) \\{{Diffusion}\mspace{14mu}{across}\mspace{14mu}{boundary}\mspace{14mu}{layer}\mspace{14mu}{to}\mspace{14mu}{the}\mspace{14mu}{substrate}\text{:}\mspace{14mu} A_{{cg},{subs}}{\prod\limits^{k_{ads}}\; A_{s,{subs}}^{*}}} & \left( {3a} \right) \\{{{Desorption}\mspace{14mu}{from}\mspace{14mu}{the}\mspace{14mu}{substrate}\mspace{14mu}{back}}\text{}{{into}\mspace{14mu}{the}\mspace{14mu}{vapor}\mspace{14mu}{phase}\text{:}\mspace{14mu} A_{s,{subs}}^{*}{\prod\limits^{k_{des}}\; A_{{cg},{subs}}}}} & \left( {3b} \right) \\{{Surface}\mspace{14mu}{diffusion}\mspace{14mu}{and}\mspace{14mu}{immobilization}\text{:}\mspace{14mu} A_{s,{subs}}^{*}{\prod\limits^{k_{c}}\; A_{s,{subs}}}} & \left( {3c} \right)\end{matrix}$where A_(s) represents an organic molecular species in the solid orliquid state and k is the rate constants representing the characteristicrate of the corresponding process. For example, species A_(s) and A_(g)evaporate and recondense inside the source cell with characteristicrates k_(evap) and k_(cond), respectively. This classificationillustrates the overall efficiency of the process in addition toascertaining the rate limiting step(s). For example, evaporation takesplace either in the so-called “kinetic” regime, where k_(evap)>k_(cond),or is in an equilibrium regime, where k_(evap)=k_(cond). The organicspecies is swept out of the source cell by the carrier gas in reaction(1c). Entrainment by the carrier results in taking A_(cg) to thevicinity of the substrate with a characteristic bulk transport rate,k_(t), where it becomes A_(cg,subs), with an overall efficiency of 100%,while the remainder is pumped out of the deposition chamber. Depositiontakes place by diffusion of A across the boundary layer and adsorptionwith a characteristic rate k_(ads). The overall deposition rate isr_(dep)=k_(dep)−k_(des), where k_(des) is the rate of desorption fromthe substrate. In the source container, the net rate of organic vaporoutflow is represented by a mass balance, where the difference betweenrates of evaporation, k_(evap), and re-condensation, k_(cond), gives therate with which organic vapor is carried out of the source cell.

U.S. Pat. No. 6,337,102 describes the basis for organic vapor phasedeposition, and is incorporated herein by reference in its entirety. Inaddition, co-pending U.S. patent application publication no.2004-0048000 A1 is incorporated herein by reference in its entirety.This co-pending U.S. patent application describes a device and methodfor organic vapor jet deposition which may be used in accordance withthe method of the present invention.

Although the organic vapor phase deposition described in U.S. Pat. No.6,337,102 employed conditions such that the resulting thin films were“characterized by superior surface properties such as low surfaceroughnesses” (U.S. Pat. No. 6,337,102, col. 2, lines 56-59), the organicvapor phase deposition of the present invention preferably employsconditions such that Stranski-Krastanov layer-plus-island growth occurs(see S. R. Forrest, Chem. Rev . 97, 1793 (1997)). That is, the organicvapor phase deposition of the present invention preferably employssufficiently high substrate temperatures and deposition chamberpressures to produce a cohesive energy of the organic small moleculematerial such that the organic small molecule material tends to adhereto itself rather than the underlying substrate. This growth regimefosters the deposition of a first layer, comprising the organic smallmolecule material, having a sufficiently high surface area-to-volumeratio to form the bottom layer of a bulk heterojunction. Preferably, thesurface area-to-volume ratio of the deposited first layer is at least2:1, and more preferably is at least 5:1. These surface area and volumemeasurements may be made by microscopic techniques known in the art,such as for example, atomic force microscopy (AFM), scanning electronmicroscopy (SEM), or cross-sectional transmission electron microscopy(TEM) (which would be the preferred technique for a completelyfabricated optoelectronic device). By increasing this surfacearea-to-volume ratio, the exciton dissociation probability, and hencethe efficiency, of the fabricated optoelectronic device are increased.As used herein in the phrase “surface area-to-volume ratio,” the term“surface area” does not refer to the entire surface area of thedeposited first layer, but refers only to the surface area of thedeposited first layer which will be in contact with the second layerdeposited thereon (i.e., the interface of the first and second layers).

The temperatures and pressures to be employed during the organic vaporphase deposition will be dependent upon, inter alia, the organicmaterials being used. For example, when depositing a first layercomprising copper phthalocyanine (CuPc) as an electron donor layer overa first electrode, a preferred deposition chamber pressure is in therange of about 50 mTorr to about 10 Torr, a preferred source celltemperature is in the range of about 370° C. to about 470° C., and apreferred underlying substrate temperature is in the range of about 0°C. to about 100° C. More preferably, the deposition chamber pressure isin the range of 0.15 to 0.80 Torr, the preferred source cell evaporationtemperature is at least 400° C., and the underlying substratetemperature is less than 40° C. The underlying substrate is thesubstrate underlying all of the deposited layers, and in practice, it isthe temperature of the underlying substrate that is controlled duringOVPD. However, because the layers deposited on the substrate are allrelatively thin, the temperatures of these layers are approximatelyequal to the temperature of the underlying substrate, such that thetemperature of the material on which the first layer is being depositedby OVPD is approximately equal to the temperature of the underlyingsubstrate. In addition, a nitrogen gas carrier flow rate in the range of10 to 200 sccm is preferred.

The use of organic vapor phase deposition to form a bulk heterojunctionduring the fabrication of an optoelectronic device offers advantagesover previous methods of forming a bulk heterojunction. For instance,OVPD enables the direct deposition of separate layers to form a bulkheterojunction on a variety of substrates, including rolls of plastic,without the need for post-deposition annealing of the fabricated device.Ordinarily, plastic substrates cannot be subjected to the hightemperatures required during post-deposition annealing. Thus, not onlydoes the use of OVPD potentially lower the costs for fabricatingoptoelectronic devices having a bulk heterojunction by eliminating theneed for post-deposition annealing, but it also allows for a broaderrange of substrates (i.e., plastic substrates) which can be used inconjunction with such fabrication.

According to another embodiment of the method of the invention, a firstlayer having protrusions is deposited over a first electrode, whereinthe first layer comprises a first organic small molecule material.According to this embodiment, the first layer may be deposited viamethods known in the art, such as vacuum deposition, vacuum thermalevaporation, spin coating, organic vapor phase deposition, inkjetprinting and other known methods. Preferably, the deposition of thefirst layer having protrusions employs conditions such thatStranski-Krastanov layer-plus-island growth occurs (see S. R. Forrest,Chem. Rev. 97, 1793 (1997)). That is, the deposition of the first layerhaving protrusions preferably employs conditions which produce acohesive energy of the first organic small molecule material such thatthe first organic small molecule material tends to adhere to itselfrather than the underlying substrate. This growth regime fosters thedeposition of a first layer, comprising the first organic small moleculematerial, having a sufficiently high surface area-to-volume ratio toform the bottom layer of a bulk heterojunction. Preferably, the surfacearea-to-volume ratio of the deposited first layer is at least 2:1, andmore preferably is at least 5:1. By increasing this surfacearea-to-volume ratio, the exciton dissociation probability, and hencethe efficiency, of the fabricated optoelectronic device are increased.

Preferably, the diameters of each of the protrusions of the depositedfirst layer are no less than about the exciton diffusion length (L_(D1))in the first organic small molecule material which comprises the firstlayer. More preferably, the diameters of each of the protrusions of thedeposited first layer are in the range of from about one to about fivetimes the exciton diffusion length (L_(D1)) of the first organic smallmolecule material. Most preferably, the diameters of each of theprotrusions of the deposited first layer are in the range of from aboutone-and-a-half to about three times the exciton diffusion length(L_(D1)) in the first organic small molecule material. These diametermeasurements may be determined by microscopic techniques known in theart, such as for example, AFM, SEM, or cross-sectional TEM (which wouldbe the preferred technique for a completely fabricated optoelectronicdevice). By depositing a first layer having protrusions with suchdiameters, the DA interface should be shaped such that thephotogenerated excitons of the first layer will likely find a DAinterface within a distance L_(D1) of their generation site. Likewise,it is preferred that the second layer deposited thereon will besimilarly shaped. That is, the protrusions of the deposited first layerwill be homogeneously arranged such that the spaces between theseprotrusions will preferably correspond to reciprocal protrusions of thedeposited second layer having diameters no less than about the excitondiffusion length (L_(D2)) in the second organic small molecule materialwhich comprises the second layer, more preferably having diameters inthe range of from about one to about five times L_(D2), and mostpreferably having diameters in the range of from about one-and-a-half toabout three times L_(D2). As used herein, the spacing betweenprotrusions is the shortest lateral distance between two adjacentprotrusions. By depositing a first layer having protrusions with such ahomogeneous arrangement, the DA interface should be shaped such that thephotogenerated excitons of the second layer will likely find a DAinterface within a distance L_(D2) of their generation site. Preferably,the height of the protrusion is at least equal to half the diameter ofthe protrusion. As used herein, the term “diameter” describes as anorder of magnitude the size of certain features of the protrusions anddoes not limit the protrusions to circular or cylindrical shapes sincethe protrusions may take various shapes. As used herein, the term“diameter” refers to the smallest lateral dimension of a feature, i.e.,the smallest dimension in a direction parallel to the plane of asubstrate on which the device is fabricated. At any point within aprotrusion, an exciton should be no more than 2.5 exciton diffusionlengths from an interface, and more preferably no more than 1.5diffusion lengths from an interface (based on the preferred diameterranges of 1-5 and 1.5-3 diffusion lengths). Consistent with the idea ofcontrolling the maximum distance that an exciton must travel to reach aninterface, it is understood that, while the cross sectional profile of aprotrusion in the lateral plane is not necessarily constant as afunction of distance in a direction perpendicular to the lateral plane,the “smallest lateral dimension” or the “diameter” of a feature ismeasured in a lateral cross section that is reasonably representative ofthe whole feature, as opposed, for example, to measuring at the tip.

Organic thin film crystal size and morphology may be controllable inOVPD. In PV devices, it is preferable to control the growth of theprotrusions of the first deposited layer, as uncontrolled growth, whichis believed to be due to excessive strain, is believed to result inshorting defects in the PV cell. In a preferred embodiment, the diameterof the protrusions of the first layer is at least 5% less than thethickness of the device. More preferably, the diameter is 10% less thanthe thickness of the device. As demonstrated herein, the growth of theprotrusions may be controlled by selection of OVPD process parameters.Relevant parameters may include the underlying substrate temperature,the source evaporation temperature, the nitrogen gas carrier flowrate,and the chamber pressure. The underlying substrate temperature isbelieved to control the growth of the needles or protrusions and isselected to sufficiently promote adsorption of the vaporized organicmaterial onto the substrate and to favor a relatively high nucleationdensity. The source evaporation temperature is dictated by the vaporpressure of the organic material and the desired deposition rate, boundon the upper end (typically above 400° C. for most organic materialsused in thin film devices) by its thermal chemical stability. It isfurther believed that a nitrogen gas carrier flowrate of 10-200 sccmresults in optimal boundary layer thickness of the donor/acceptorinterface facilitating transport of the organic material. Chamberpressure is generally limited by the pump employed in the OVPD systemand also depends on the gas carrier flowrate. It is believed that thepreferred range of 0.15-0.80 Torr sufficiently promotes the acceptormaterial filling the gaps and recesses of the underlying donor film, asshown in FIG. 12. Depending upon the materials used and the desiredresults, parameters outside of these ranges may be used in certaincircumstances.

According to the method of embodiments of the invention, after the firstlayer comprising a first organic small molecule material has beendeposited over the first electrode, a second layer is then deposited onthe first layer such that the second layer is in physical contact withthe first layer, and the interface of the second layer on the firstlayer forms a bulk heterojunction. The second layer may be deposited viavacuum deposition, vacuum thermal evaporation, spin coating, organicvapor phase deposition, inkjet printing and other methods known in theart. Preferably, the top surface of the top organic layer, which may bethe second layer, is relatively flat such that shunt-paths for theelectrical current are avoided. It is believed that deposition by OVPDwith selected process parameters promotes a planarized top film surfaceover other deposition techniques, namely VTE. In VTE, the deposited filmtypically has a morphology that conforms to the underlying surface (seeFIG. 10), resulting from the ballistic trajectories followed by theincident molecules. Hence, the acceptor material surface would followthat of the donor material needles, also resulting in a second, highlyfolded needle array. In addition, voids are likely to develop where theprotrusions shadow the underlying surface of the film from the acceptormaterial source cell. This is also shown in FIG. 11, where a VTE grownPTCBI/BCP/Ag layer exhibits a high density of voids at the bases of thetallest CuPc needles. In contrast, surface diffusion coupled to therandom arrival directions as the molecules traverse the boundary layerat the substrate surface in OVPD growth is believed to result in aplanarized top film surface with complete filling of the spaces beneaththe needles in the underlying CuPc, as shown in FIGS. 12 and 13.

When the first layer is deposited by OVPD, it is preferred that thesecond layer is deposited on the first layer also by OVPD. As would beunderstood by one of ordinary skill in the art, the temperatures andpressures to be employed during the OVPD of the second layer will bedependent upon the organic materials being used. For example, whendepositing (by OVPD) a second layer comprising3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI) as anelectron acceptor layer on a first layer comprising copperphthalocyanine (CuPc) as an electron donor layer, a preferred depositionchamber pressure is in the range of about 50 mTorr to about 10 Torr, apreferred source cell temperature is in the range of about 370° C. toabout 470° C., and a preferred underlying substrate temperature is inthe range of about 0° C. to about 100° C. More preferably, thedeposition chamber pressure is in the range of 0.15 to 0.80 Torr, thepreferred source cell evaporation temperature is at least 400° C., andthe underlying substrate temperature is less than 40° C. As thesepreferred temperature and pressure ranges are the same as those employedfor depositing a first layer comprising CuPc as an electron donor layerover a first electrode as previously described, such an OVPD processcould be carried out in the same deposition chamber, which would help toexpedite and economize the fabrication process.

Although layers of any thickness may be used in accordance with themethod of the invention, several guidelines should be considered in theselection of preferred layer thicknesses when fabricating anoptoelectronic device according to the method of the invention. It isdesirable for the exciton diffusion length, L_(D), to be greater than orcomparable to the layer thickness, L, since it is believed that mostexciton dissociation will occur at an interface. If L_(D) is less thanL, then many excitons may recombine before dissociation. It is furtherdesirable for the total photoconductive layer thickness to be of theorder of the electromagnetic radiation absorption length, 1/∀(where ∀ isthe absorption coefficient), so that nearly all of the radiationincident on the solar cell is absorbed to produce excitons. Furthermore,the photoconductive layer thickness should be as thin as possible toavoid excess series resistance due to the high bulk resistivity oforganic semiconductors.

Accordingly, these competing guidelines inherently require tradeoffs tobe made in selecting the thicknesses of the photoconductive organiclayers of a photosensitive optoelectronic device. Thus, on the one hand,a thickness that is comparable or larger than the absorption length isdesirable (for a single cell device) in order to absorb the maximumamount of incident radiation. On the other hand, as the photoconductivelayer thickness increases, two undesirable effects are increased. One isthat due to the high series resistance of organic semiconductors, anincreased organic layer thickness increases device resistance andreduces efficiency. Another undesirable effect is that increasing thephotoconductive layer thickness increases the likelihood that excitonswill be generated far from the effective field at a charge-separatinginterface, resulting in enhanced probability of geminate recombinationand, again, reduced efficiency. Therefore, a device configuration isdesirable which balances between these competing effects in a mannerthat produces a high quantum efficiency for the overall device. Forexample, by using multiple stacked subcells, the photoconductive organiclayers may be made very thin. In summary, by taking the above-notedcompeting effects into account, that is, the absorption length of thephotoconductive materials in the device, the diffusion length of theexcitons in these materials, the photocurrent generation efficiency ofthese excitons, and the resistivity of these materials, the thickness ofthe layers in a device may be adjusted so as to obtain a maximuminternal quantum efficiency (and those parameters directly relatedthereto) for those particular materials for a given set of ambientradiation conditions.

As previously described, the interface of the second layer on the firstlayer forms a bulk heterojunction which produces an internally generatedelectric field. Preferably, the bulk heterojunction is formed by anelectron donor layer in physical contact with an electron acceptorlayer. The material comprising the electron donor layer has anionization potential that is smaller than that of the materialcomprising the electron acceptor layer. Furthermore, the ionizationpotential HOMO/LUMO gap of the electron donor layer must be smaller thanthat of the electron acceptor layer. Generally, the materials comprisingthe electron donor or electron acceptor layers should have the longestpossible exciton diffusion length, and thus are preferably thosematerials which lend themselves to ordered stacking of the molecules,such as planar, aromatic molecules.

The electron acceptor layer may be comprised of, for example, perylenes,naphthalenes, fullerenes or nanotubules. A preferred electron acceptormaterial is 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI).Alternatively, the electron acceptor layer may be comprised of afullerene material, such as buckminsterfullerene (C₆₀), as described inU.S. Pat. No. 6,580,027, which is incorporated herein by reference inits entirety.

The electron donor layer may be comprised of, for example, apthalocyanine or a porphyrin, or a derivative or transition metalcomplex thereof. A preferred electron donor material is copperpthalocyanine (CuPc).

According to an embodiment of the method of the invention, a secondelectrode is deposited over the second layer to form the optoelectronicdevice. The second electrode may be deposited via methods known in theart, such as for example, vacuum deposition and spin coating. Theelectrodes, or contacts, used in a photosensitive optoelectronic deviceare an important consideration, as shown in U.S. Pat. No. 6,352,777,which is incorporated herein by reference in its entirety. When usedherein, the terms “electrode” and “contact” refer to layers that providea medium for delivering photogenerated power to an external circuit orproviding a bias voltage to the device. That is, an electrode, orcontact, provides the interface between the photoconductively activeregions of an organic photosensitive optoelectronic device and a wire,lead, trace or other means for transporting the charge carriers to orfrom the external circuit. In a photosensitive optoelectronic device, itis desirable to allow the maximum amount of ambient electromagneticradiation from the device exterior to be admitted to thephotoconductively active interior region. That is, the electromagneticradiation must reach a photoconductive layer, where it can be convertedto electricity by photoconductive absorption. This often dictates thatat least one of the electrical contacts should be minimally absorbingand minimally reflecting of the incident electromagnetic radiation. Thatis, such a contact should be substantially transparent. The opposingelectrode may be a reflective material so that light which has passedthrough the cell without being absorbed is reflected back through thecell. As used herein, a layer of material or a sequence of severallayers of different materials is said to be “transparent” when the layeror layers permit at least 50% of the ambient electromagnetic radiationin relevant wavelengths to be transmitted through the layer or layers.Similarly, layers which permit some, but less that 50% transmission ofambient electromagnetic radiation in relevant wavelengths are said to be“semi-transparent”.

The electrodes are preferably composed of metals or “metal substitutes.”Herein the term “metal” is used to embrace both materials composed of anelementally pure metal, e.g., Mg, and also metal alloys which arematerials composed of two or more elementally pure metals, e.g., Mg andAg together, denoted Mg:Ag. Herein, the term “metal substitute” refersto a material that is not a metal within the normal definition, butwhich has the metal-like properties that are desired in certainappropriate applications. Commonly used metal substitutes for electrodesand charge transfer layers would include doped wide-bandgapsemiconductors, for example, transparent conducting oxides such asindium tin oxide (ITO), gallium indium tin oxide (GITO), and zinc indiumtin oxide (ZITO). In particular, ITO is a highly doped degenerate n+semiconductor with an optical bandgap of approximately 3.2 eV, renderingit transparent to wavelengths greater than approximately 3900 Å. Anothersuitable metal substitute is the transparent conductive polymerpolyanaline (PANI) and its chemical relatives. Metal substitutes may befurther selected from a wide range of non-metallic materials, whereinthe term “non-metallic” is meant to embrace a wide range of materialsprovided that the material is free of metal in its chemically uncombinedform. When a metal is present in its chemically uncombined form, eitheralone or in combination with one or more other metals as an alloy, themetal may alternatively be referred to as being present in its metallicform or as being a “free metal.” Thus, the metal substitute electrodesof the present invention may sometimes be referred to as “metal-free”wherein the term “metal-free” is expressly meant to embrace a materialfree of metal in its chemically uncombined form. Free metals typicallyhave a form of metallic bonding that results from a sea of valenceelectrons which are free to move in an electronic conduction bandthroughout the metal lattice. While metal substitutes may contain metalconstituents they are “non-metallic” on several bases. They are not purefree-metals nor are they alloys of free-metals. When metals are presentin their metallic form, the electronic conduction band tends to provide,among other metallic properties, a high electrical conductivity as wellas a high reflectivity for optical radiation.

Embodiments of the method of the present invention may include, as oneor more of the transparent electrodes of the fabricated optoelectronicdevice, a highly transparent, non-metallic, low resistance cathode suchas disclosed in U.S. Pat. Nos. 6,469,437, and 6,420,031 to Parthasarathyet al. (“Parthasarathy”), or a highly efficient, low resistancemetallic/non-metallic compound cathode such as disclosed in U.S. Pat.Nos. 5,703,436, and 6,297,495 to Forrest et al. (“Forrest”). Each typeof cathode is preferably prepared in a fabrication process that includesthe step of sputter depositing an ITO layer onto either an organicmaterial, such as copper phthalocyanine (CuPc), to form a highlytransparent, non-metallic, low resistance cathode or onto a thin Mg:Aglayer to form a highly efficient, low resistance metallic/non-metalliccompound cathode. Parasarathy discloses that an ITO layer onto which anorganic layer had been deposited, instead of an organic layer onto whichthe ITO layer had been deposited, does not function as an efficientcathode.

Herein, the term “cathode” is used in the following manner. In anon-stacked photosensitive optoelectronic device or a single unit of astacked photosensitive optoelectronic device under ambient irradiationand connected with a resistive load and with no externally appliedvoltage, e.g., a solar cell, electrons move to the cathode from theadjacent photoconducting material. Similarly, the term “anode” is usedherein such that in a solar cell under illumination, holes move to theanode from the adjacent photoconducting material, which is equivalent toelectrons moving in the opposite manner. It will be noted that as theterms are used herein, anodes and cathodes may be electrodes or chargetransfer layers.

In another embodiment of the invention, the optoelectronic device may befabricated over a substrate. For example, the first electrode may beover a substrate, such that when the first layer is deposited over thefirst electrode, the first layer is also deposited over the substrate.The substrate may be any suitable substrate that provides desiredstructural properties. The substrate may be flexible or rigid, and itmay also be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. The substratemay be a semiconductor material in order to facilitate the fabricationof circuitry. For example, the substrate may be a silicon wafer uponwhich circuits are fabricated, capable of controlling optoelectronicdevices subsequently deposited on the substrate. Other substrates mayalso be used. The material and thickness of the substrate may be chosento obtain desired structural and optical properties.

In a further embodiment of the invention, the method of fabricating anoptoelectronic device may include depositing one or more excitonblocking layers (EBLs) as described in U.S. Pat. Nos. 6,097,147, and6,451,415; and in Peumans et al., Applied Physics Letters 2000, 76,2650-52, all of which are incorporated herein by reference in theirentireties. The EBL may be deposited via vacuum deposition, vacuumthermal evaporation, spin coating, organic vapor phase deposition,inkjet printing and other methods known in the art. Higher internal andexternal quantum efficiencies have been achieved by the inclusion of oneor more EBLs to confine photogenerated excitons to the region near thedissociating interface and to prevent parasitic exciton quenching at aphotosensitive organic/electrode interface. In addition to limiting thevolume over which excitons may diffuse, an EBL can also act as adiffusion barrier to substances introduced during deposition of theelectrodes. In some circumstances, an EBL can be made thick enough tofill pinholes or shorting defects which could otherwise render anorganic PV device non-functional. An EBL can therefore help protectfragile organic layers from damage produced when electrodes aredeposited onto the organic materials.

It is believed that the EBLs derive their exciton blocking property fromhaving a LUMO-HOMO energy gap substantially higher than that of theadjacent organic semiconductor from which excitons are being blocked.Thus, the confined excitons are prohibited from exiting from the EBL dueto energy considerations. While it is desirable for the EBL to blockexcitons, it is not desirable for the EBL to block all charge. However,due to the nature of the adjacent energy levels, an EBL will necessarilyblock only one sign of charge carrier. By design, an EBL will alwaysexist between two layers, usually an organic photosensitivesemiconductor layer and an electrode or charge transfer layer. Theadjacent electrode or charge transfer layer will be in context either acathode or an anode. Therefore, the material for an EBL in a givenposition in a device will be chosen so that the desired sign of carrierwill not be impeded in its transport to the electrode or charge transferlayer. Proper energy level alignment ensures that no barrier to chargetransport exists, preventing an increase in series resistance. Forexample, it is desirable for a material used as a cathode side EBL tohave a LUMO level closely matching the LUMO level of the adjacentacceptor material so that any undesired barrier to electrons isminimized.

It should be appreciated that the exciton blocking nature of a materialis not an intrinsic property. Whether a given material will act as anexciton blocker depends upon the relative HOMO and LUMO levels of theadjacent organic photosensitive material. Therefore, it is not possibleto identify a class of compounds in isolation as exciton blockerswithout regard to the device context in which they may be used. However,with the teachings herein one of ordinary skill in the art may identifywhether a given material will function as an exciton blocking layer whenused with a selected set of materials to construct an organic PV device.

In a preferred embodiment of the invention, an EBL is deposited betweenthe electron acceptor layer and the cathode. A preferred material forthe EBL comprises 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (alsocalled bathocuproine or BCP), which is believed to have a LUMO-HOMOseparation of about 3.5 eV, orbis(2-methyl-8-hydroxyquinolinoato)-aluminum(III)phenolate (Alq₂OPH).BCP is an effective exciton blocker which can easily transport electronsto the cathode from the adjoining electron acceptor layer. Furthermore,if BCP is deposited by OVPD, a preferred source temperature for the BCPwould be in the range of about 150° C. to about 250° C.

Embodiments of the method of the invention may also include thedepositing of semi-transparent to transparent charge transfer layers. Asdescribed herein charge transfer layers are distinguished from donor andacceptor layers by the fact that charge transfer layers are frequently,but not necessarily, inorganic and they are generally chosen not to bephotoconductively active. The term “charge transfer layer” is usedherein to refer to layers similar to but different from electrodes inthat a charge transfer layer only delivers charge carriers from onesubsection of an optoelectronic device to the adjacent subsection.

In a further embodiment of the method of the invention, one or more ofthe deposited layers may be treated with plasma prior to depositing thenext layer. The layers may be treated, for example, with a mild argon oroxygen plasma. This treatment is beneficial as it reduces the seriesresistance.

In another embodiment of the method of the invention, a wetting layer isdeposited over the first electrode, such that the first layer isdeposited over the wetting layer (i.e., the wetting layer is between thefirst electrode and the first layer), and a planarizing layer isdeposited over the second layer, such that the second electrode isdeposited over the planarizing layer (i.e., the planarizing layer isbetween the second layer and the second electrode). The deposition ofthe wetting layer and the planarizing layer help to prevent pinholeformation in the thin films of the fabricated optoelectronic device.When depositing the second electrode on top of the previously depositedlayers, electrical shorts can result due to the penetration of thesecond electrode into the pinholes and contact with the underlying firstelectrode. However, the deposition of the wetting layer and theplanarizing layer help to prevent such electrical shorts from occurring.An exemplary material for the planarizing layer and the wetting layer isa film of 3,4-polyethylenedioxythiophene:polystyrenesulfonate(PEDOT:PSS).

According to this embodiment of the invention, the wetting layer and theplanarizing layer may be deposited by methods known in the art, such asvacuum deposition, vacuum thermal evaporation, spin coating, organicvapor phase deposition, inkjet printing and other known methods. Whenemploying OVPD to deposit the wetting layer and the planarizing layer,lower substrate temperatures and deposition chamber pressures than thoseused to deposit the first layer may be used.

Generally, the high bulk resistivities of organic photoconductors makeit desirable to utilize relatively thin films of these materials whenfabricating organic optoelectronic devices. However, thin photosensitivelayers will absorb a smaller fraction of incident radiation, and thusthe external quantum efficiency of thin-layer photoconductors may belower than that of thick-layer photoconductors. The external quantumefficiency of thin-layer organic devices such as those described hereincan be further enhanced, however, by a suitable design of the devicegeometry. Due to the thin photoactive layers of the devices describedherein, device geometries which provide a means for increasing theeffective thickness of the absorbant layers may be preferable. One suchconfiguration is a stacked device as described in U.S. Pat. No.6,657,378, which is incorporated herein by reference in its entirety. Itshould be understood that the method of the present invention can beused to fabricate stacked optoelectronic devices, such as for examplethose described in U.S. Pat. No. 6,657,378. As used herein, the terms“stack”, “stacked”, “multisection” and “multicell” refer to anyoptoelectronic device with multiple layers of a photoconductive materialseparated by one or more electrode or charge transfer layers. When theterm “subcell” is used herein, it refers to an organic photosensitiveoptoelectronic construction. When a subcell is used individually as aphotosensitive optoelectronic device, it typically includes a completeset of electrodes, i.e., positive and negative. In a stackedoptoelectronic device, each of the subcells in the device may beelectrically connected either in parallel or in series, depending onwhether the current and/or voltage generated by the device is to bemaximized. In some stacked configurations it is possible for adjacentsubcells to utilize common, i.e., shared, electrode or charge transferlayers. In other cases, adjacent subcells do not share common electrodesor charge transfer layers, but are instead separated by an electron-holerecombination zone comprising, for example, metallic nanoclusters,nanoparticles or nanorods. Thus, a subcell may encompass the subunitconstruction regardless of whether each subunit has its own distinctelectrodes or shares electrodes or charge transfer layers with adjacentsubunits. Herein, the terms “cell”, “subcell”, “unit”, “subunit”,“section”, and “subsection” are used interchangeably to refer to aphotoconductive-layer or set of layers and the adjoining electrodes orcharge transfer layers.

As described above, in some embodiments of stacked configurations, theindividual subcells of the stacked devices are separated by anelectron-hole recombination zone, such as disclosed in U.S. Pat. No.6,657,378, which is incorporated herein by reference in its entirety. Asused herein, “front” means closest to the incident electromagneticradiation and “back” means further from the electromagnetic radiationsource. The electron-hole recombination zone serves to prevent theformation of an inverse heterojunction between the exciton-blockinglayer of the front cell and the donor layer of the back cell. It isunderstood that one could combine any number of subcells in this manner.The effective recombination of electrons from the front subcell andholes from the back subcell is necessary if a photo-induced current isto be achieved in the stacked device. Preferably, the electron-holerecombination zone comprises a thin metal layer. The metal issemi-transparent to transparent in order to allow light to reach theback cell(s). To this end, it is preferred that the metal layer be lessthan about 20 Δ thick. It is especially preferred that the metal film beabout 5 Δ thick. It is believed that these ultra-thin metal films (˜5Δ)are not continuous films but rather are composed of isolated metalnanoparticles, nanorods or nanoclusters. Surprisingly, although theultra-thin metal layer is not continuous, it still provides an efficientlayer for electron-hole recombination. Preferred materials for use inthis layer include Ag, Li, LiF, Al, Ti, and Sn. Silver is a particularlypreferred metal for this layer. Gold is not believed to be a good choicefor this layer as it is not known to introduce mid-gap states. In oneembodiment of a stacked device, the electron-hole recombination zonecomprises a region of electronically active defects which lead to rapidelectron-hole recombination. The defects may be introduced by limiteddamage at this interface, for example, by heating, by controlledimpurity introduction, or by exposure to energetic particles during thedeposition of the relevant organic layers. The energetic particles maybe excited, for example, thermally or by an RF plasma.

FIG. 4 shows an optoelectronic device 400 produced by an embodiment ofthe method of the present invention. Device 400 may include a substrate410, a first electrode 420, a first layer 430, a second layer 440, and asecond electrode 450. This figure is not necessarily drawn to scale. Forexample, although the bulk heterojunction which is formed by theinterface of the second layer 440 on the first layer 430 is representedby a solid line in FIG. 4, the bulk heterojunction is more accuratelydepicted by the schematic diagram of FIG. 1( b). A representativeembodiment of the device 400 includes an anode as the first electrode420, an electron donor layer as the first layer 430, an electronacceptor layer as the second layer 440, and a cathode as the secondelectrode 450. Another representative embodiment of the device 400includes a cathode as the first electrode 420, an electron acceptorlayer as the first layer 430, an electron donor layer as the secondlayer 440, and an anode as the second electrode 450.

The simple layered structure illustrated in FIG. 4 is provided by way ofnon-limiting example, and it is understood that embodiments of theinvention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional devices may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations or a mixture of materials may be used.Also, the layers may have various sublayers. The names given to thevarious layers herein are not intended to be strictly limiting. Forexample, in one embodiment of device 400, first layer 430 transportselectrons to the bulk heterojunction in the interface of layer 430 and440, and may be described as an electron donor layer or a electrontransporting layer. Moreover, the order of the layers may be altered.For example, the cathode may be on top in some embodiments and the anodein other embodiments. The order of the organic layers in theseembodiments may be altered accordingly.

According to other embodiments of the method of the invention, the firstand second electrodes need not be included, and the deposition of thefirst and second layers as described above could be done over only asubstrate. That is, in another embodiment of the invention, a method offorming a bulk heterojunction comprises depositing a first layer over asubstrate by organic vapor phase deposition, wherein the first layercomprises a first organic small molecule material; and depositing asecond layer on the first layer such that the second layer is inphysical contact with the first layer, wherein the interface of thesecond layer on the first layer forms a bulk heterojunction. In afurther embodiment of the invention, a method of forming a bulkheterojunction comprises depositing a first layer having protrusionsover a substrate, wherein the first layer comprises a first organicsmall molecule material; and depositing a second layer on the firstlayer such that the second layer is in physical contact with the firstlayer, wherein the interface of the second layer on the first layerforms a bulk heterojunction.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. In addition, the method ofthe present invention may include other known processing steps used inthe fabrication of optoelectronic devices. Also, it is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

Experimental

Specific representative embodiments of the invention will now bedescribed. It is understood that the specific methods, materials,conditions, process parameters, apparatus and the like do notnecessarily limit the scope of the invention.

EXAMPLE 1

Organic vapor phase deposition was employed to deposit CuPc (an electrondonor material) on silicon (a substrate) at an underlying substratetemperature of about 60° C. The deposited layer of CuPc had a highsurface area-to-volume ratio, as can be seen in the scanning electronmicrograph shown in FIG. 3( a).

EXAMPLE 2

Organic vapor phase deposition was employed to deposit CuPc (an electrondonor material) on silicon (a substrate) at an underlying substratetemperature of about 100° C. The deposited layer of CuPc had a highsurface area-to-volume ratio, as can be seen in the scanning electronmicrograph shown in FIG. 3( b).

EXAMPLE 3

Organic vapor phase deposition was employed to deposit CuPc (an electrondonor material) on indium-tin-oxide (an electrode) at an underlyingsubstrate temperature of about 100° C. The deposited layer of CuPc had ahigh surface area-to-volume ratio, as can be seen in the scanningelectron micrograph shown in FIG. 3( c).

EXAMPLE 4

Organic vapor phase deposition was employed to deposit CuPc onto asilicon substrate. The source evaporation temperature was at 440±5° C.,a substrate temperature at 100±5° C., a nitrogen carrier gas flow rateof 30.0±0.1 sccm, and a chamber pressure of 0.45±0.01 Torr. The growthduration was 10 minutes. FIG. 5 shows a scanning electron microscope(SEM) image of the Stranski-Krastonov island-plus-layer growth modegenerating the combination of a thin but continuous wetting layer plusshort needles. The dense wetting layer is believed to prevent directcontact between the acceptor material and the anode.

EXAMPLE 5

Organic vapor phase deposition was employed to deposit CuPc onto anindium-tin-oxide coated electrode under the same conditions as Example4. FIG. 6 shows a scanning electron microscope image of the surface of aCuPc film.

EXAMPLE 6

Organic vapor phase deposition was employed to deposit CuPc onto anindium-tin-oxide coated electrode. This structure containing a very highdensity of short needles is suitable as the bottom layer of a bulkheterojunction. The source evaporation and substrate temperatures arefixed at 425±5° C. and 100±5° C., respectively. The nitrogen carrier gasflow rate increases linearly from 14 sccm to 200 sccm during the 6.5-mingrowth duration, and chamber pressure rises from 0.18 Torr to 0.70 Torr.The inset shows the AFM image of the same sample. The root mean square(RMS) surface roughness is 2.7 nm. FIG. 7 shows the densely distributedshort needles with an average length of 35 nm and a diameter of 30 nm.

EXAMPLE 7

Vacuum thermal evaporation was employed to deposit CuPC onto anindium-tin-oxide coated electrode. FIG. 8 shows the surface of the 500thick CuPc film. The inset shows the AFM image of the sample with an RMSroughness of 0.8 nm. Scale bars are all 500 nm.

The x-ray diffraction spectrum of the OVPD CuPc film in Example 6 isshown in FIG. 9( a), and is similar to that of the Example 7 film grownby vacuum thermal evaporation as shown in FIG. 9( b). The diffractionpeak at 2θ=6.7° confirms the existence of the β-CuPc phase. The domainsize is calculated to be 20±5 nm from the full width at half maximum(FWHM) of the diffraction peak, from which it is inferred that eachsmall protrusion in FIG. 7 is a single CuPc microcrystalline domain.

EXAMPLE 8

Growth of the CuPc in Example 6 is followed by that of a 500±25 Å thickPTCBI layer without exposure of the interface to atmosphere. To ensure aflat surface, the source material is heated to 462±3° C., while thesubstrate is maintained at 16±5° C. The N₂ flow rate is fixed at 150±1sccm for 10.4 minutes, resulting in a chamber pressure of 0.58±0.01Torr. A 100-Å thick BCP exciton blocking and electron conducting layerand a 1000-Å thick Ag 1 mm² circular cathode contact deposited through ashadow mask are grown by conventional vacuum thermal evaporation tocomplete the PV cell. FIG. 13 shows an SEM image of the planarizedsurface. The inset shows a cross-section of the PV device, which has thefollowing layers: glass, ITO, CuPc/PTCBI/BCP (700 Å), Ag (700 Å). Thescale bar is 500 nm.

COMPARATIVE EXAMPLE

An ITO(450 Å)/CuPc(500 Å)/PTCBI(100 Å)/BCP(1000 Å)/Ag device having aplanar heterojunction was made for comparison. The planar CuPc layer isdeposited for 3 minutes with a source temperature of 445±3° C., asubstrate temperature of 3±5° C., and a N₂ flow rate of 100±1 sccm. ThePTCBI is deposited under the same conditions as used for the bulkheterojunction device.

TABLE I Comparison of performance of several ITO/CuPc/PTCBI/BCP/Agphotovoltaic cell structures^(a) J_(sc) R_(SA) (mA/cm²) V_(OC)(V) FFη_(P) (%) (Ω cm²) Planar VTE HJ 6 0.49 0.49 1.1 ± 0.1 30 ± 10 Annealed 90.50 0.40 1.4 ± 0.1 60 ± 10 Bulk HJ Planar OVPD HJ 5 0.48 0.47 1.1 ± 0.118.2 ± 0.5  Controlled Bulk 11 0.49 0.58 2.7 ± 0.1 2.2 ± 0.1 OVPD HJ^(a)The illumination intensity is 1 sun (100 mW/cm²) simulated AM1.5Gfor all 4 devices.

The current density-voltage characteristics of the bulk doubleheterojunction PV cell as a function of illumination intensity are shownin FIG. 14. A performance comparison between this bulk heterojunctionand other double heterojunction CuPc/PTCBI PV cells is summarized inTable I. The short circuit current is approximately twice that obtainedfor a vacuum deposited or OVPD planar heterojunction PV cell.Furthermore it is approximately 20% higher than for a previouslyreported “thermodynamically driven” bulk heterojunction demonstrated byannealing a mixed CuPc/PTCBI layer. The dependence of the photocurrenton reverse bias of the grown bulk heterojunction is considerably reducedfrom that observed for the annealed device. Fitting the forward-biasdark current density according to the modified ideal diode equation:

$\begin{matrix}{{J_{D} = {J_{S}\left\{ {{\exp\left\lbrack \frac{q\left( {V - {J_{D}R_{SA}}} \right)}{nkT} \right\rbrack} - 1} \right\}}},} & (1)\end{matrix}$leads to a series resistance of R_(SA)=(2.2±0.1)Ωcm², which is smallerthan the value obtained from a random bulk heterojunction device whereR_(SA)=(60±10)Ωcm². This suggests that series resistance due to theamorphous growth, or bottlenecks to carrier collection has been reducedin the OVPD-grown bulk heterojunction cell.

The dependence of the performance characteristics on the simulated AM1.5 G solar illumination intensity are shown in FIG. 15. Theopen-circuit voltage is V_(OC)=(0.50±0.03)V at 1 sun (100 mW/cm²)intensity. Given that this is a characteristic of the materials systemsemployed, V_(OC) is approximately the same for all the CuPc/PTCBI cellscompared in Table I. Other suitable materials in these devices mayexhibit open-circuit voltages, V_(OC), of about 0.4 to about 1 V andcurrent densities, J_(SC), greater than 9 mA/cm². FIG. 15 shows that fordevices with similar V_(OC), the fill factor of the OVPD-grown,controlled bulk heterojunction device is FF>0.5 for illumination overthe range of 0.01-10 suns (1-1000 mW/cm²). The combination of highphotocurrent density and FF results in a high external power conversionefficiency (η_(P)), with a maximum of η_(P)=(2.7±0.1)% at 100 mW/cm².

Referring to Table I, η_(P) is approximately 2.5 times higher thanachieved using a comparable planar HJ PV cell, and 1.9 times higher thanan annealed, thermodynamically driven bulk heterojunction, suggestingthat the controlled growth of the CuPc/PTCBI bulk heterojunction resultsin the desired increase in junction surface area without introducing anincreased cell series resistance.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1. A method of fabricating an optoelectronic device, comprising:depositing a first layer having protrusions over a first electrode,wherein the first layer comprises a first organic small moleculematerial; depositing a second layer on the first layer such that thesecond layer is in physical contact with the first layer and theinterface of the second layer and the first layer forms a bulkheterojunction; wherein the smallest lateral dimension of theprotrusions is between 1 to 5 times the exciton diffusion length of thefirst organic small molecule material; and depositing a second electrodeover the second layer to form the optoelectronic device.
 2. The methodof claim 1, wherein the second layer comprises polymer or semiconductormaterial.
 3. The method of claim 2, wherein the spacing betweenprotrusions is between 1 to 5 times the exciton diffusion length of thesecond layer.
 4. The method of claim 1, wherein the second layercomprises a metal.
 5. The method of claim 1, wherein the height of theprotrusions is at least equal to half of the smallest lateral dimensionof the protrusions.
 6. The method of claim 1, wherein: the first layerhaving protrusions is deposited over the first electrode by organicvapor phase deposition, and the smallest lateral dimension of theprotrusions is at least 5% less than the thickness of the device; andthe second layer has a planar surface.
 7. The method of claim 6, whereinthe first layer is an electron acceptor layer, the first electrode is acathode, the second layer is an electron donor layer, and the secondelectrode is an anode.
 8. The method of claim 6, further comprisingdepositing the first layer over a substrate.
 9. The method of claim 6,wherein the device has a current density, J_(SC), greater than 9(mA/cm²) and an open circuit voltage, V_(OC), between 0.4 volts and 1volts.
 10. The method of claim 6, wherein the device has a seriesresistance of less than 10 Ωcm².
 11. The method of claim 6, wherein thedevice has an external power conversion efficiency of at least 1.5%under 100 mW/cm² (AM 1.5 G) illumination.
 12. The method of claim 6,further comprising: depositing a charge recombination layer over thesecond layer; depositing a third layer having protrusions over thesecond electrode by organic vapor phase deposition, wherein the thirdlayer comprises a third organic small molecule material and the smallestlateral dimension of the protrusions is at least 5% less than thethickness of the device; depositing a fourth layer on the third layersuch that the fourth layer is in physical contact with the third layer,wherein the interface of the fourth layer and the third layer forms abulk heterojunction, and the fourth layer has a planar surface; anddepositing a third electrode over the second layer to form theoptoelectronic device.
 13. The method of claim 6, wherein the firstlayer is an electron donor layer, the first electrode is an anode, thesecond layer is an electron acceptor layer, and the second electrode isa cathode.
 14. The method of claim 13, wherein the first electrodecomprises ITO, the first layer comprises CuPc, and the second layercomprises PTCBI.
 15. The method of claim 14, wherein the sourceevaporation temperature for deposition of the first layer is at least400° C.
 16. The method of claim 14, wherein the underlying substratetemperature for deposition of the first layer is less than 40° C. 17.The method of claim 14, wherein the nitrogen gas carrier flow rate fordeposition of the first layer is from 10-200 sccm.
 18. The method ofclaim 14, wherein the chamber pressure for deposition of the first layeris from 0.15-0.80 Torr.
 19. The method of claim 14, wherein the sourceevaporation temperature for deposition of the second layer is at least400° C.
 20. The method of claim 14, wherein the substrate temperaturefor deposition of the second layer is less than 25° C.
 21. The method ofclaim 14, wherein the nitrogen gas carrier flow rate for deposition ofthe first layer is at least 100 sccm.
 22. The method of claim 14,wherein the chamber pressure is from at least 0.50 Torr.
 23. The methodof claim 13, further comprising depositing a third layer over the secondlayer, such that the second electrode is deposited over the third layer.24. The method of claim 23, wherein the third layer is an excitonblocking layer.
 25. The method of claim 24, wherein the third layercomprises BCP.
 26. The method of claim 23, further comprising:depositing an electron-hole recombination zone over the third layer;depositing a fourth layer over the electron-hole recombination zone byorganic vapor phase deposition, wherein the fourth layer comprises afourth organic small molecule material; depositing a fifth layer on thefourth layer such that the fifth layer is in physical contact with thefourth layer, wherein the interface of the fifth layer on the fourthlayer forms a bulk heterojunction, and the fifth layer has a planarsurface; and depositing an exciton blocking layer over the fifth layer;and, depositing the second electrode over the fifth layer to form theoptoelectronic device.
 27. The method of claim 13, wherein the firstelectrode comprises ITO, the first layer comprises CuPc, and the secondlayer comprises C₆₀.